Solubility, dissolution and permeability are important factors in the screening, evaluation, production or administration of pharmaceutical compounds. The Biopharmaceutics Classification System (BCS), for example, uses these parameters to characterize drug substances into four groups. Highly permeable and highly soluble compounds (Class I) are well absorbed, with typical absorption rates higher than excretion. For high permeability, low solubility drugs (Class II) bioavailability is limited by the solubility and/or dissolution rate. Drugs that belong in Class III are characterized by low permeability and high solubility, typically exhibiting an absorption limited by the permeation rate. Low permeability, low solubility compounds (Class IV) tend to be poorly absorbed over the intestinal mucosa and often exhibit high variability in pharmaco-kinetic (PK) studies.
In many cases, solubility, dissolution and permeability of solid oral doses are studied individually. Dissolution (sometimes also referred to as “release”) characteristics, for example, are routinely tested in vitro, in the preliminary stages of drug development, or to ensure batch to batch quality control during the manufacturing process. Techniques for correlating laboratory results to in vivo behavior as well as standardized equipment and protocols for dissolution testing have been developed. The United States Pharmacopeia (USP), for example, describes several systems, including USP Dissolution Apparatus I—Basket (37° C.) and USP Dissolution Apparatus II—Paddle (37° C.).
Laboratory methods for investigating permeability often rely on membranes that mimic in vivo systems. Human colon carcinoma epithelial cell line Caco-2 cell monolayers, for example, are routinely used as a model of the human small intestinal mucosa. Permeability across a membrane such as a Caco-2 cell monolayer can then be correlated to in vivo absorption according to suitable protocols.
A frequently used approach in permeability studies is the “parallel artificial membrane permeability assay” or PAMPA. In this approach, a compound is studied as it permeates from a donor compartment, through a lipid-infused filter support constituting artificial membrane into an acceptor compartment. In traditional PAMPA, a multi-well microtitre plate (hence “parallel”) is used for the donor and a membrane/acceptor compartment is placed on top; the whole assembly is often referred to as a “sandwich”. A PAMPA test is usually initiated by adding the pre-dissolved drug under investigation to the donor compartment, and providing a drug-free acceptor compartment. After an incubation period which may include stirring, in one or both compartments, the sandwich is separated and the amount of drug in each compartment may be measured. Drug amounts retained in the membrane can be determined using mass balance calculations or other techniques.
Though generally PAMPA cannot measure active permeability (a form of transport that uses cellular energy and relies on the movement of molecules across a cell membrane that not necessarily aligned with the concentration gradient, e.g., moving from a low concentration to a high concentration), PAMPA can provide valuable information regarding passive transport, i.e., a movement of atoms or molecules (e.g., biochemicals) across cell membranes that does not require an input of chemical energy but is driven by an increase in the entropy of the system. The rate of passive transport expressed by the permeability constant depends in part on the interaction between the permeating molecule and the barrier membrane, which, in turn, depends on the organization and characteristics of the membrane lipids and proteins.
In many cases, PAMPA is used in the early screenings of active pharmaceutical ingredients (API), while Caco-2 studies are used in later stages of drug development.
Several attempts to combine dissolution and permeability (typically using Caco-2 monolayers) assessments also have been undertaken. An article by M. J. Ginski et al., with the title Prediction Of Dissolution-Absorption Relationships From A Continuous Dissolution/Caco-2 System, AAPS Pharm Sci 1: E3 (1999), for instance, describes an arrangement in which a USP apparatus II (rotating paddle) is used as a dissolution vessel. The dissolution chamber is linked to a permeation chamber containing a donor compartment (cell) and a receiver (absorption) compartment (cell) in a side-by-side arrangement. A peristaltic pump is used to transfer medium from the dissolution vessel to the donor compartment and permeation occurs through a Caco-2 cell monolayer separating the two cells.
A more complex approach, proposed by M. Kobayashi et al., Development Of A New System For Prediction Of Drug Absorption That Takes Into Account Drug Dissolution And pH Change In The Gastro-Intestinal Tract, Int. J. Pharm. 221: 87-94 (2001), employs separate vessels for dissolution, pH adjustment and permeation.
Yet another arrangement is described by M. Kataoka et al. in the paper In Vitro System 70 Evaluate Oral Absorption Of Poorly Water-Soluble Drugs: Simultaneous Analysis On Dissolution And Permeation Of Drugs, Pharm. Res. 20: 1674-1680 (2003). The approach involves a downsized vessel containing a Caco-2 monolayer between an apical side and a basal side. The drug to be tested is introduced to the apical side and dissolution profiles are established by blocking the Caco-2 monolayer with a flat aluminum sheet. Permeation is measured by taking aliquots from the basal side.
Other designs rely on a single compartment, containing a layer of 1-octanol above a water layer, to simultaneously study the release and partitioning of a drug in the two phases.